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Peter Dahl

Senior Principal Engineer

Professor, Mechanical Engineering

Email

dahl@apl.washington.edu

Phone

206-543-2667

Research Interests

Underwater Acoustics, Acoustic Remote Sensing

Biosketch

Dr. Dahl is a Senior Principal Engineer in the Acoustics Department and a Professor in the University of Washington's Department of Mechanical Engineering. Professor Dahl's research is in areas of acoustics with primary focus on underwater sound. Examples of his research include underwater acoustic remote sensing, the acoustics of underwater explosions, acoustic scattering and reflection from the sea surface and sea bed, vector acoustics, underwater ambient noise and methods to reduce underwater industrial noise.

He has conducted several ocean-going experiments involving underwater acoustics, including the Asian Seas International Acoustics Experiment (ASIAEX), sponsored by the U.S. Office of Naval Research, in the East China Sea involving the U.S., China and Korea and for which he was U.S. chief scientist.

Professor Dahl is a Fellow of the Acoustical Society of America, has served as the chair of its technical committee on underwater acoustics (2002–2005), on its Executive Council (2008–2011), and has recently completed service as Vice President of the Acoustical Society of America.

Department Affiliation

Acoustics

Education

Ph.D. Ocean Engineering, Massachusetts Institute of Technology, 1989

Publications

2000-present and while at APL-UW

On the underwater sound field from impact pile driving: Arrival structure, precursor arrivals, and energy streamlines

Dahl, P.H., and D.R. Dall'Osto, "On the underwater sound field from impact pile driving: Arrival structure, precursor arrivals, and energy streamlines," J. Acoust. Soc. Am., 142, 1141, doi:10.1121/1.4999060, 2017.

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1 Aug 2017

Underwater noise from impact pile driving is studied through measurements using a vertical line array (VLA) placed at range 120 m from the pile source (water depth 7.5 m) over which bathymetry varied gradually increasing to depth 12.5 m at the VLA. The data were modeled assuming the pile impact produces a radial expansion that acts as sound source and propagates along the pile at supersonic speed. This leads to the conceptualization of the pile as a discrete, vertical line source for which frequency- and source-depth-dependent complex phasing is applied. Dominant features of the pressure time series versus measurement depth are reproduced in modeled counterparts that are linearly related. These observations include precursor arrivals for which arrival timing depends on hydrophone depth and influence of a sediment sound speed gradient on precursor amplitude. Spatial gradients of model results are taken to obtain estimates of acoustic particle velocity and vector intensity for which active intensity is studied in the time domain. Evaluation of energy streamlines based on time-integrated active intensity, and energy path lines based on instantaneous (or very-short-time integrated) active intensity reveal interesting structure in the acoustic field, including an inference as to the source depth of the precursor.

Observations of sea-surface waves during the 2013 Target and Reverberation Experiment (TREX13) and relation to midfrequency sonar

Dahl, P.H., and D.R. Dall'Osto, "Observations of sea-surface waves during the 2013 Target and Reverberation Experiment (TREX13) and relation to midfrequency sonar," IEEE J. Ocean. Eng., EOR, doi:10.1109/JOE.2016.2597718, 2016.

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15 Sep 2016

As part of the 2013 Target and REverberation eXperiment (TREX13), which took place off the coast of Panama City, FL, USA, directional wave measurements were made using two directional wave buoys separated in range by 5 km. The purpose of these measurements was to provide environmental support for the interpretation of reverberation and other active sonar experiments that were part of TREX13. During the measurement period between April 22 and May 17, 2013 exclusive of a period of nondeployment May 2–6, 2013, the root-mean-square (rms) wave height H varied over the range 0.03–0.33 m, holding a median value of 0.11 m; the wind speed varied from ~1 to 10 m/s with a median value of 4.7 m/s, and the rms wave slope averaged over all directions varied from 0.01 to 0.10 with median value of 0.05. These parameters are placed in the context of midfrequency sonar propagation and reverberation prediction. One buoy operated the entire period, with the second buoy operating simultaneously over a four-day overlap period, during which there was excellent agreement between H and wave slope in two orthogonal directions, a finding relevant to describing the sea surface as spatially invariant, or homogeneous, for purposes of sonar modeling. The analysis of energy-weighted mean direction illustrates how the wave field was generally composed of a mixture of swell and wind-generated waves; in cases of purely wind-generated waves the effect of a limited fetch was also shown.

Measurement of acoustic particle motion in shallow water and its application to geoacoustic inversion

Dall'Osto, D.R., C.W. Choi, and P.H. Dahl, "Measurement of acoustic particle motion in shallow water and its application to geoacoustic inversion," J. Acoust. Soc. Am., 139, 311-319, doi:/10.1121/1.4939492, 2016

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15 Jan 2016

Within an underwater acoustic waveguide, the interference among multipath arrivals causes a phase difference in orthogonal components of the particle velocity. When two components of the particle velocity are not in phase, the fluid particles follow an elliptical trajectory. This property of the acoustic field can be readily detected by a vector sensor. A non-dimensional vector quantity, the degree of circularity, is used to quantify how much the trajectory resembles a circle. In this paper, vector sensormeasurements collected during the 2013 Target and Reverberation Experiment are used to demonstrate the effect of multipath interference on the degree of circularity. Finally, geoacoustic properties representing the sandy sediment at the experimental site are inverted by minimization of a cost function, which quantifies the deviation between the measured and modeled degree of circularity.

More Publications

Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed

Soloway, A.G., P.H. Dahl, and R.I. Odom, "Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed," J. Acoust. Soc. Am., 138, EL370-374, doi:10.1121/1.4931831, 2015

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9 Oct 2015

Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1%u2009m from the source as a function of TNT-equivalent charge weight.

The underwater sound field from vibratory pile driving

Dahl, P.H., D.R. Dall'Osto, and D.M. Farrell, "The underwater sound field from vibratory pile driving," J. Acoust. Soc. Am., 137, 3544-3554, doi:10.1121/1.4921288, 2015.

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1 Jun 2015

Underwater noise from vibratory pile driving was observed using a vertical line array placed at range 16 m from the pile source (water depth 7.5 m), and using single hydrophones at range 417 m on one transect, and range 207 and 436 m on another transect running approximately parallel to a sloping shoreline. The dominant spectral features of the underwater noise are related to the frequency of the vibratory pile driving hammer (typically 15–35 Hz), producing spectral lines at intervals of this frequency. The mean-square pressure versus depth is subsequently studied in third-octave bands. Depth and frequency variations of this quantity observed at the vertical line array are well modeled by a field consisting of an incoherent sum of sources distributed over the water column. Adiabatic mode theory is used to propagate this field to greater ranges and model the observations made along the two depth-varying transects. The effect of shear in the seabed, although small, is also included. Bathymetric refraction on the transect parallel to the shoreline reduced mean-square pressure levels at the 436-m measurement site.

The underwater sound field from impact pile driving and its potential effects on marine life

Dahl, P.H., C.A.F. de Jong, and A.N. Popper, "The underwater sound field from impact pile driving and its potential effects on marine life," Acoust. Today, 11, 18-25, 2015.

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1 Jun 2015

Impact pile driving is a method used to install piles for marine and inland water construction projects using high-energy impact hammers. The installation of hollow steel piles in this manner can produce extremely high sound levels in the surrounding waters (as well as in the air). Given the large-scale development of offshore wind in European waters and plans for such development in US waters, along with an increasing need for upgrades in the in-water infrastructure, there is a growing concern about the potential effect of construction-related underwater sounds on marine mammal and fish populations.

Six decades of evolution in underwater acoustics at the Applied Physics Laboratory, University of Washington

Williams, K.L. D. Tang, P.H. Dahl, E.I. Thorns, D.R. Jackson, and T.E. Ewart, "Six decades of evolution in underwater acoustics at the Applied Physics Laboratory, University of Washington," J. Acoust. Soc. Am., 137, 2331, doi:10.1121/1.4920514, 2015

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1 Apr 2015

Professor Joe Henderson of the University of Washington physics department formed the Applied Physics Laboratory during WWII. The lab’s initial efforts were to redesign the magnetic influence exploders used in US torpedoes. One of the lab’s first Underwater Acoustics (UA) successes was development of transducers used in the Bikini Atoll Able test (1946). Those transducers, used to trigger other instrumentation, proved critical. Combining UA and torpedo expertise brought APL-UW to the forefront of tracking range design, construction and deployment in Dabob Bay, Nanoose, and St. Croix in the 1950s and 1960. Understanding the torpedo behavior seen in tracking ranges required measuring both the ocean environment and the acoustics within that environment. Making those measurements, as well as development and testing of models based on those measurements, also became standard operating procedure at APL, led in the 50’s by Murphy and Potter. This blueprint of applied research motivating basic research, and the pursuit of basic research via ocean experiments and high fidelity modeling, continues to this day. The presentation will follow this evolution. APL-UW ocean experiments carried out during that time, as well as notable APL-UW research papers, technical reports, computer codes and textbooks, will be used as guideposts.

Peak sound pressure and sound exposure level from underwater explosions in shallow water

Soloway, A.G., and P.H. Dahl, "Peak sound pressure and sound exposure level from underwater explosions in shallow water," J. Acoust. Soc. Am., 136, EL218, doi:10.1121/1.4892668, 2014.

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1 Sep 2014

Experimental measurements of the peak pressure and sound exposure level (SEL) from underwater explosions collected 7–9km off the coast of Virginia Beach, Virginia are presented. The peak pressures are compared to results from previous studies and a semi-empirical equation that is a function of measurement range and charge weight, and are found to be in good agreement. An empirical equation for SEL that similarly employs a scaling approach involving charge weight and range is also presented and shows promise for the prediction of SEL in shallow water.

Vertical coherence and forward scattering from the sea surface and the relation to the directional wave spectrum

Dahl, P.H., W.J. Plant, and D.R. Dall'Osto, "Vertical coherence and forward scattering from the sea surface and the relation to the directional wave spectrum," J. Acoust. Soc. Am., 134, 1843-1853, doi:10.1121/1.4817846, 2013.

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1 Sep 2013

Results of an experiment to measure vertical spatial coherence from acoustic paths interacting once with the sea surface but at perpendicular azimuth angles are presented. The measurements were part of the Shallow Water 2006 program that took place off the coast of New Jersey in August 2006. An acoustic source, frequency range 6–20 kHz, was deployed at depth 40 m, and signals were recorded on a 1.4-m long vertical line array centered at depth 25 m and positioned at range 200 m. The vertical array consisted of four omni-directional hydrophones and vertical coherences were computed between pairs of these hydrophones. Measurements were made over four source–receiver bearing angles separated by 90°, during which sea surface conditions remained stable and characterized by a root-mean-square wave height of 0.17 m and a mixture of swell and wind waves. Vertical coherences show a statistically significant difference depending on source–receiver bearing when the acoustic frequency is less than about 12 kHz, with results tending to fade at higher frequencies. This paper presents field observations and comparisons of these observations with two modeling approaches, one based on bistatic forward scattering and the other on a rough surface parabolic wave equation utilizing synthetic sea surfaces.

Elliptical acoustic particle motion in underwater waveguides

Dall'Osto, D., and P.H. Dahl, "Elliptical acoustic particle motion in underwater waveguides," J. Acoust. Soc. Am., 134, 109-118, doi:10.1121/1.4807747, 2013.

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1 Jul 2013

Elliptical particle motion, often encountered in acoustic fields containing interference between a source signal and its reflections, can be quantified by the degree of circularity, a vector quantity formulated from acoustic particle velocity, or vector intensity measurements. Acoustic analysis based on the degree of circularity is expected to find application in ocean waveguides as its spatial dependence relates to the acquisition geometry, water column sound speed, surface conditions, and bottom properties. Vector sensor measurements from a laboratory experiment are presented to demonstrate the depth dependence of both the degree of circularity and an approximate formulation based on vertical intensity measurements. The approximation is applied to vertical intensity field measurements made in a 2006 experiment off the New Jersey coast (in waters 80 m deep) to demonstrate the effect of sediment structure on the range dependence of the degree of circularity. The mathematical formulation presented here establishes the framework to readily compute the degree of circularity from experimental measurements; the experimental examples are provided as evidence of the spatial and frequency dependence of this fundamental vector property.

Beam forming of the underwater sound field from impact pile driving

Dahl, P.H., and P.G. Reinhall, "Beam forming of the underwater sound field from impact pile driving," J. Acoust. Soc. Am., 134, EL1-EL6, doi:10.1121/1.4807430, 2013.

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5 Jun 2013

Observations of underwater noise from impact pile driving were made with a vertical line array. Previous studies [Reinhall and Dahl, J. Acoust. Soc. Am. 130, 1209–1216 (2011)] show that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile at supersonic speed after impact. Here precise estimates of the vertical arrival angles associated with the down- and up-going Mach wave are made via beam forming, and the energy budget of the arrival structure is quantified.

Underwater radiated noise measurements of a noise-reduced fisheries research vessel

De Robertis, A., C.D. Wilson, S.R. Furnish, and P.H. Dahl, "Underwater radiated noise measurements of a noise-reduced fisheries research vessel," ICES J. Mar. Sci., 70, 480-484, doi:10.1093/icesjms/fss172, 2013.

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1 Mar 2013

Vessel-radiated noise is traditionally measured at naval acoustic ranges, but lower-cost options are desirable for routine monitoring of research vessels. Measurements of a noise-reduced research vessel made at a naval noise range are compared to those made using an experimental mooring equipped with commercially available instrumentation. The measurements from the mooring were precise and within 2.5 dB of those from the noise range at third-octave bands <500 Hz. At higher frequencies, direct comparisons were precluded by an intermittent shaft-related noise present only during the mooring measurements, but previously observed at the navy range. The agreement between the two methods suggests that simplified, field-deployable hydrophone systems can be used to accurately characterize the noise signatures of research vessels.

Attenuation of pile driving noise using a double walled sound shield

Reinhall, P.G., and P.H. Dahl, "Attenuation of pile driving noise using a double walled sound shield," J. Acoust. Soc. Am., 132, 2034, doi:10.1121/1.4755475, 2012.

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1 Sep 2012

Pile driving in water produces high sound levels in underwater environments. The associated pressures are known to produce deleterious effects on both fish and marine mammals. We present an evaluation of the effectiveness of surrounding the pile with a double walled sound shield to decrease impact pile driving noise. Four 32 m long, 76 cm diameter piles were driven 14 m into the sediment with a vibratory hammer. A double walled sound shield was then installed around the pile, and the pile was impact driven another 3 m while sound measurements were obtained. The last 0.3 m was driven with the sound shield removed, and data were collected for the untreated pile. The sound field obtained by finite element analysis is shown to agree well with measure data. The effectiveness of the sound shield is found to be limited by the fact that an upward moving Mach wave is produced in the sediment after the first reflection of the deformation wave against the bottom end of the pile. The sound reduction obtained through the use of the sound shield, as measured 10 meters away from the pile, is shown to be approximately 12dB dB re 1 µPa.

Modeling and visualization of the underwater sound field associated with underwater pile driving

Farrell, D.M., and P.H. Dahl, "Modeling and visualization of the underwater sound field associated with underwater pile driving," J. Acoust. Soc. Am., 132, 2061, doi:10.1121/1.4755595, 2012.

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1 Sep 2012

As communities seek to expand and upgrade marine and transportation infrastructure, underwater noise from pile driving associated with marine construction is a significant environmental regulatory challenge. This work explores results of different transmission loss models for a site in Puget Sound and the effect of improved understanding of modeling on the extents of zones of influence. It has been observed that most of the energy associated with impact pile driving is less than about 1000 Hz. Here, analysis of the spectral content of pile driving noise is undertaken to ascertain the optimal surrogate frequency to model the broadband nature of the noise. Included is a comparison of a normal mode model, which is motivated by work presented by Reinhall and Dahl [JASA 130, 1209 (2011)], with other methods. A GIS (Geographic Information System) tool, ArcMap, is used to map the sound level over the bathymetry, which has proved to be a useful way of visualizing the impact of the noise.

On the Mach wave effect in impact pile driving, its observation, and its influence on tramsmission loss

Dahl, P.H., and P.G. Reinhall, "On the Mach wave effect in impact pile driving, its observation, and its influence on tramsmission loss," J. Acoust. Soc. Am., 132, 2033, doi:10.1121/1.4755474, 2012.

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1 Sep 2012

Pile driving in water produces extremely high sound pressure levels in the surrounding underwater environment of order 10 kPa at ranges of order 10 m from the pile that can result in deleterious effects on both fish and marine mammals. In Reinhall and Dahl [J. Acoust. Soc. Am. 130, 1209-1216, Sep. 2011] it is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile at speeds in excess of Mach 3 with respect to the underwater sound speed. In this talk we focus on observations of the Mach wave effect made with a 5.6 m-length vertical line array, at ranges 8-15 m in waters of depth ~12.5 m. The key observation is the dominant vertical arrival angle associated with the Mach wave, ~17 deg., but other observations include: its frequency dependence, the ratio of purely waterborne energy compared with that which emerges from the sediment, and results of a mode filtering operation which also points to the same dominant angle. Finally, these observations suggest a model for transmission loss which will also be discussed.

The effect of bottom layering on the acoustic vector field

Dall'Osto, D.R., and P.H. Dahl, "The effect of bottom layering on the acoustic vector field," J. Acoust. Soc. Am., 132, 2092, doi:10.1121/1.4755735, 2012.

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1 Sep 2012

A signal reflected from a layered sea-bed contains information pertaining to the sediment properties. Typically, a signal intended to probe the sea-bed is designed to have a large bandwidth to allow for time separation of arrivals from the multiple layers. Depending on the geometry, it may impossible to avoid interference of these arrivals. The interference of these multiple arrivals does establish a pattern observable in the vector intensity. Measurements of the vertical complex acoustic intensity of a near-bottom source (~λ from the seafloor) collected off the coast of New Jersey in 2006 demonstrate the effect of a sub-bottom layer and the observable interference pattern between the first bottom reflection and the sub-bottom reflection. The spatial structure of the complex intensity can be used to infer bottom properties, which are in close agreement with a number of experimental studies at this location. The observable in the complex intensity can also be directly measured with a particle motion sensor. Parabolic equation simulations of the experimental site are used to demonstrate both the characteristic of the vector field and the sensitivity of these vector properties to changes in the sediment properties.

Underwater vector intensity measurements in the ocean and laboratory

Dall'Osto, D.R., and P.H. Dahl, "Underwater vector intensity measurements in the ocean and laboratory," J. Acoust. Soc. Am., 132, 1985, doi:10.1121/1.4755327, 2012.

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1 Sep 2012

Underwater measurements of the acoustic intensity vector field can be provided by either spatially separated hydrophones or by a sensor measuring a property of particle motion, such as particle acceleration. These measurements are used to formulate the vector intensity as the product of pressure and particle velocity. The magnitude of the vector intensity is not necessarily equal to the plane-wave intensity (the mean square pressure divided by the density and sound-speed of the medium) which is often used to define pressure measurements in terms of intensity. In regions of strong destructive interference, the magnitude of the vector intensity may be greater than the plane-wave intensity. Measurements of an impulsive source on a vertical line array of pressure sensors spanning a shallow sea (60 m) off the coast of South Korea are presented to demonstrate properties of the complex intensity vector field in an ocean waveguide. Here, the vertical complex intensity is formulated by finite-difference methods. These vertical intensity observations in the ocean waveguide have implications on properties of the complete vector field. A laboratory experiment using a tri-axial particle acceleration sensor is presented to provide a connection between measurement of elliptical particle motion and complex intensity.

Attenuation of noise from pile driving in water using an acoustic shield

Reinhall, P., and P. Dahl, "Attenuation of noise from pile driving in water using an acoustic shield," Proceedings, 11th European Conference on Underwater Acoustics, 2-6 July, Edinburgh, 368-375 (Institute of Acoustics, 2012).

2 Jul 2012

Observations of underwater sound from impact pile driving using a vertical line array

Dahl, P., and P. Reinhall, "Observations of underwater sound from impact pile driving using a vertical line array," Proceedings, 11th European Conference on Underwater Acoustics, 2-6 July, Edinburgh, 1340-1347 (Institute of Acoustics, 2012).

2 Jul 2012

Transmission loss and range, depth scales associated with impact pile driving

Dahl, P., P. Reinhall, and D. Farrell, "Transmission loss and range, depth scales associated with impact pile driving," Proceedings, 11th European Conference on Underwater Acoustics, 2-6 July, Edinburgh, 1860-1867 (Institute of Acoustics, 2012).

2 Jul 2012

Waveguide properties of active intensity vorticity

Dall'Osto, D., and P. Dahl, "Waveguide properties of active intensity vorticity," Proceedings, 11th European Conference on Underwater Acoustics, 2-6 July, Edinburgh, 1939-1947 (Institute of Acoustics, 2012).

2 Jul 2012

Scattering measurements from a dissolving bubble

Kapodistrias, G., and P.H. Dahl, "Scattering measurements from a dissolving bubble," J. Acoust. Soc. Am., 131, 4243-4251, doi:10.1121/1.3703060, 2012.

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1 Jun 2012

A laboratory-scale study on acoustic scattering from a single bubble undergoing dissolution in undersaturated fresh water is presented. Several experiments are performed with the acoustic source driven with five-cycle tone bursts, center frequency of 120 kHz, to insonify a single bubble located on axis of the combined beam of the set of transducers. The bubble is placed on a fine nylon thread located in the far field of the transducer set, arranged in bistatic configuration, in a tank filled with undersaturated water. Backscattered waveforms from the bubble target are acquired every few seconds for several hours until the bubble has completely dissolved, and detailed dissolution curves are produced from the acoustic data. The rate of bubble dissolution is calculated using the solution developed by Epstein and Plesset [J. Chem. Phys. 18, 1505-1509 (1950)]. The results of the experiments performed are in agreement with the calculations.

Properties of the acoustic intensity vector field in a shallow water waveguide

Dall'Osto, D.R., P.H. Dahl, and J.W. Choi, "Properties of the acoustic intensity vector field in a shallow water waveguide," J. Acoust. Soc. Am., 131, 2023-2035, doi:10.1121/1.3682063, 2012.

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1 Mar 2012

Acoustic intensity is a vector quantity described by collocated measurements of acoustic pressure and particle velocity. In an ocean waveguide, the interaction among multipath arrivals of propagating wavefronts manifests unique behavior in the acoustic intensity. The instantaneous intensity, or energy flux, contains two components: a propagating and non-propagating energy flux. The instantaneous intensity is described by the time-dependent complex intensity, where the propagating and non-propagating energy fluxes are modulated by the active and reactive intensity envelopes, respectively.

Properties of complex intensity are observed in data collected on a vertical line array during the transverse acoustic variability experiment (TAVEX) that took place in August of 2008, 17 km northeast of the Ieodo ocean research station in the East China Sea, 63 m depth. Parabolic equation (PE) simulations of the TAVEX waveguide supplement the experimental data set and provide a detailed analysis of the spatial structure of the complex intensity. A normalized intensity quantity, the pressure-intensity index, is used to describe features of the complex intensity which have a functional relationship between range and frequency, related to the waveguide invariant. The waveguide invariant is used to describe the spatial structure of intensity in the TAVEX waveguide using data taken at discrete ranges.

Underwater Mach wave radiation from impact pile driving: Theory and observation

Reinhall, P.G., and P.H. Dahl, "Underwater Mach wave radiation from impact pile driving: Theory and observation," J. Acoust. Soc. Am., 130, 1209-1216, doi:10.1121/1.3614540, 2011

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1 Sep 2011

The underwater noise from impact pile driving is studied using a finite element model for the sound generation and parabolic equation model for propagation. Results are compared with measurements using a vertical line array deployed at a marine construction site in Puget Sound. It is shown that the dominant underwater noise from impact driving is from the Mach wave associated with the radial expansion of the pile that propagates down the pile after impact at supersonic speed. The predictions of vertical arrival angle associated with the Mach cone, peak pressure level as function of depth, and dominant features of the pressure timeseries compare well with corresponding field observations.

Acoustic radiation during marine pile driving

Reinhall, P.G. and P.H. Dahl, "Acoustic radiation during marine pile driving," J. Acoust. Soc. Am., 129, 2460, doi:10.1121/1.3588091, 2011.

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1 Apr 2011

Pile driving in water produces extremely high sound levels in the surrounding under water environment. Sound levels as high as 220 dB re 1 micro Pa are not uncommon 10 m away from a steel pile as it is driven into the sediment with an impact hammer. The primary source of underwater sound originating from pile driving is associated with compression of the pile. The pile is struck and the Poisson effect produces a radial displacement motion in the pile that will propagate downward at a computed speed comparable to but less than the longitudinal wave speed in steel. It is shown, using both finite element analysis and modeling based on the parabolic wave equation, that this radial motion of the pile is responsible for the ensuing high underwater sound pressures. It is also shown that the radial motion of the pile is transmitted into the water, either directly from the pile or indirectly via the bottom sediment that is in contact with the pile. A dominant feature of the resulting sound field is an axisymmetric Mach cone with apex traveling along with the pile deformation wave front.

Acoustic transmission loss in industrial pile driving

Stockham, M.L., P.H. Dahl, and P.G. Reinhall, "Acoustic transmission loss in industrial pile driving," J. Acoust. Soc. Am., 129, 2461, doi:10.1121/1.3588094, 2011.

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1 Apr 2011

Industrial pile driving is a source of high-level underwater noise and new understanding of its effects on the behavior and health of marine mammals and fish has motivated numerous regulations intended to limit these effects. Of primary importance is to identify a suitable transmission loss model to predict where, given a certain source level, the noise produced by the pile driving reaches regulatory thresholds. In November 2009, data were collected from a marine construction site in the Puget Sound. Measurements at two ranges (8 and 12 m) from the pile being driven were taken using a nine hydrophone vertical line array (VLA). Concurrently, at a range of approximately 120 m, there was also a single hydrophone at a depth of 5 m (sensitive to frequencies greater than 10 kHz). By comparing the levels at the VLA to the more distant hydrophone across a number of pile strikes (each forming a identifiable short- and far-range pair), the transmission loss can be estimated. These results are in turn modeled using an approach based on the parabolic wave equation.

Airborne noise contributions to the underwater noise sound field

Dall'Osto, D.R., and P.H. Dahl, "Airborne noise contributions to the underwater noise sound field," J. Acoust. Soc. Am., 129, 2498, doi:10.1121/1.3588252, 2011.

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1 Apr 2011

Contributions of airborne noise sources to the underwater noise field are the result of two acoustic fields: the transmitted and evanescent. The transmitted field can be represented by only those rays confined to a small cone (about 26 deg) where the reflection coefficient is real-valued. The evanescent field, which arises when rays are totally reflected from the surface, can also contribute to the underwater noise field. Unlike the transmitted field, the evanescent field does not propagate and decays exponentially with depth with a decay rate as a function of frequency. Determining the individual contribution of these two fields to the overall sound field is experimentally difficult to observe. One situation where these two fields can be observed individually occurs when an airplane flies overhead. The Doppler shift associated with tonal propeller noise is dependent on the acoustic path. The frequency separation of the two fields allows for separate analysis of the two fields. Measurements from aircraft (altitude 1000 ft) passing over a buoy equipped with a microphone 3 m above the surface and a hydrophone 2.5 m below the surface will be presented. Numerical simulations are presented along with the experimental observations.

Observations and parabolic wave modeling of underwater pile driving impact noise

Dahl, P.H., and P.G. Reinhall, "Observations and parabolic wave modeling of underwater pile driving impact noise," J. Acoust. Soc. Am., 129, 2461, doi: 10.1121/1.3588093, 2011.

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1 Apr 2011

Pile driving in water produces extremely high sound levels in both surrounding air and underwater environments. In a companion work [Reinhall and Dahl] it is shown using finite element simulation that for underwater case the primary sound signal originates from a compression wave traveling down the pile at a speed in excess of Mach 3. In this work, we present measurements pile driving impact noise made from a marine construction site in Puget Sound using a vertical line array (VLA) positioned at ranges 815 m from full-scale impact pile driving. The measurements are modeled using the parabolic wave equation approach for which synthetic time series are generated (bandwidth 50-2050 Hz). The simulation is achieved by way of a phased array of point sources, representing one source traveling down the pile at supersonic speed. Pile end reflections are included and the process is repeated with both an up- and down-traveling time-delayed sources. With the field computed in this manner, excellent agreement is achieved between model and observations of peak pressure level, and the compression wave speed is also confirmed by way of arrival angle estimation using the VLA. Implications on transmission loss are also discussed.

Underwater radiated noise measurements of a noise-reduced research vessel: Comparison between a U.S. Navy noise range and a simple hydrophone mooring

De Robertis, A., C.D. Wilson, and P.H. Dahl, "Underwater radiated noise measurements of a noise-reduced research vessel: Comparison between a U.S. Navy noise range and a simple hydrophone mooring," J. Acoust. Soc. Am., 129, 2462, doi:10.1121/1.3588099, 2011.

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1 Apr 2011

A feasibility study was undertaken to characterize underwater radiated noise for a new class of noise-reduced fisheries research vessels using a field-deployable hydrophone system. Recent studies have demonstrated that vessel-radiated noise can impact the behavior of fish, and that periodic monitoring of survey-vessel radiated noise is desirable to characterize potential biases in fish abundance estimates. Vessel radiated noise is traditionally measured at naval ranges, but lower-cost options are desirable. Beam aspect measurements of a noise-reduced vessel made at a U.S. Navy noise range are compared to those made using an experimental mooring equipped with commercially available instrumentation. Hydrophone depths and distance-to-the-vessel were comparable for the mooring and those used at the Southeast Alaska Acoustic Measurement Facility (SEAFAC). SEAFAC and mooring measurements were taken within a day of one another. Data processing was consistent with the recent American national standard for measurement of underwater sound from ships (ANSI-ASA S12.64-2009-Part 1). The measurements from the experimental mooring were precise and comparable to those made at SEAFAC. This suggests that reliable measurements suitable for monitoring the underwater radiated noise of vessels with low source levels can be made in the field.

Vertical intensity structure in a shallow water waveguide

Dall'Osto, D.R., P.H. Dahl, and J.W. Choi, "Vertical intensity structure in a shallow water waveguide," J. Acoust. Soc. Am., 129, 2601, doi:10.1121/1.3588624, 2011.

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1 Apr 2011

Acoustic intensity in an ocean waveguide is described by the local pressure and particle velocity, both of which can be described as a sum of modes. Analysis of the interaction between these modal components gives insight into the formation of characteristic intensity structures, such as interference patterns. Observations of the modal structure of the pressure and vertical velocity in a shallow water waveguide are presented using experimental data from an experiment off Korean coastal waters, the transverse acoustic variability experiment (TAVEX) that took place in August of 2008 17 km northeast of the Ieodo weather station, in waters 62 m deep. Mode filtering is performed on a 16 element vertical array that spans the water column (3 m spacing) for broadband (imploding light bulb) sources detonated at ranges from 200 to 1000 m at 40 m depth. The vertical velocity field, determined through the finite-difference approximation, and the pressure field at 230 Hz are represented by six propagating modes and their corresponding modal amplitudes. The interaction of these modal components is analyzed and PE simulated data are presented for comparison. Nondimensional indices are formulated relating the modal components of vector intensity and their utility as field indicators will be discussed.

Observations of 0th order head waves in the Yellow Sea

Dahl, P.H., and J.W. Choi, "Observations of 0th order head waves in the Yellow Sea," In Proceedings, Second International Shallow-Water Acoustics Conference (SWAC'09), Shanghai, 16-20 September 2009, 133-139 (AIP, 2010).

4 Oct 2010

Acoustic radiation from a submerged pile during pile driving

Reinhall, P.G., and P.H. Dahl, "Acoustic radiation from a submerged pile during pile driving," In Proceedings, MTS/IEEE OCEANS 2010, Seattle, 20-23 September, doi:10.1109/OCEANS.2010.5663872 (MTS/IEEE, 2010).

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20 Sep 2010

Pile driving with an impact hammer is inherently a transient process and can produce very high sound levels. It is shown that the underwater noise during pile driving is due to a radial expansion of the pile that propagates along the pile after impact. This structural wave produces a wave front cone in the water, and a downward moving wave that continues into the sediment. An upward moving wave front is produced in the sediment after the first reflection of the structural wave, which is subsequently transmitted into the water. This process is repeated to produce an acoustic field that consists of wave fronts with alternating positive and negative angles. Good agreement in the estimate of the angles was obtained between a finite element wave propagation model and measurements taken during a full scale pile driving study.

Characterizing underwater noise from industrial pile driving at close range

Stockham, M.L., P.H. Dahl, and P.G. Reinhall, "Characterizing underwater noise from industrial pile driving at close range," In Proceedings, MTS/IEEE OCEANS 2010, Seattle, 20-23 September, doi:10.1109/OCEANS.2010.5663899 (MTS/IEEE, 2010).

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20 Sep 2010

Acoustic radiation in the water surrounding a ferry terminal pile driving project has been characterized based on 1/3-octave band filtering. The pressure field at approximately 10 meters is shown to be depth dependent based on data captured using a vertical line array. Decibel measurements of underwater sound are often expressed in reference to several non-interchangeable units, a summary of the differences of these references is presented.

Implications of signal intensity fluctuations on vector sensor array processing

Dall'Osto, D.R., and P.H. Dahl, "Implications of signal intensity fluctuations on vector sensor array processing," In Proceedings, MTS/IEEE OCEANS 2010, Seattle, 20-23 September, doi:10.1109/OCEANS.2010.5663783 (MTS/IEEE, 2010).

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20 Sep 2010

Vector sensor processing relies on the covariance matrix for both a single vector sensor and a larger matrix from a vector sensor array. The elements of these covariance matrices have physical interpretation in terms of complex intensity. The presence of reactive intensity on the array shows up in the off diagonal elements of the covariance matrix and has significant implications on direction of arrival (DOA) algorithms. Sources of reactive intensity in an underwater waveguide are dependent on the geometry of the system and fluctuations in these quantities affect the ability to increase the array aperture to better resolve arrival angles.

Observations and modeling of angular compression and vertical spatial coherence in sea surface forward scattering

Dahl, P.H., "Observations and modeling of angular compression and vertical spatial coherence in sea surface forward scattering," J. Acoust. Soc. Am., 127, 96-103, doi:10.1121/1.3268594, 2010.

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1 Jan 2010

Measurements and modeling of spatial coherence and related angular spreading associated with forward scattering from the sea surface are presented. The measurements were taken in waters 80 m deep off the New Jersey coast in August 2006. Acoustic signals from a source at depth 40 m were recorded on a vertical line array of length 1.4 m, centered at depth 25 m, and at range 200 m. Measurements in the 14–20-kHz frequency range are reported; the rms waveheight H was 0.16 m, setting kH as ~10, where k is acoustic wavenumber. A systematic study of measurements taken over four source–receiver bearing angles separated by 90 deg suggests a null influence of changing bearing angle or equivalently directional wave effects. Sound speed was characterized by a downward-refracting profile. Refraction modifies the vertical angular spread due to rough sea surface scattering, which can be understood from Snell's law. The Snell mapping is smooth, so an approximation based on the mean grazing angle provides a functional relation between the angular variance near the surface and that at the receiver. The latter is measurably reduced owing to refraction, the effect called angular compression, and a parameter that quantifies this effect is defined.

The evolution of vertical spatial coherence with range from source

Dahl. P.H., D. Tang, and J.W. Choi, "The evolution of vertical spatial coherence with range from source," J. Acoust. Soc. Am., 125, 2511, 2009.

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1 Apr 2009

Vertical spatial coherence for shallow water propagation at frequencies 1–10 kHz is studied as function of range (50 to 5000 m), as part of the Shallow-Water 2006 program that took place off the coast of New Jersey in August 2006 in waters 80 m deep. An acoustic source was deployed from the R/V Knorr at depths 30 and 40 m and signals were recorded on a moored receiving system consisting of two 1.4 m long vertical line arrays (VLA) centered at depths 25 and 50 m. At all ranges, spatial coherence (normalized spatial correlation) is locally stationary and depends on element vertical separation d up to the maximum kd (59) afforded by the VLA, where k is acoustic wave number. For range normalized by depth, r*, less than about 10, spatial coherence is oscillatory, with non-zero imaginary part, reflecting the inclusion of multipaths for which no single path dominates. For r* greater than 10, spatial coherence tends to exhibit a monotonic decay with kd and the imaginary part vanishes reflecting symmetry about 0 deg vertical arrival angle. The coherence also increases with r* reflecting the change in modal structure.

On scattering effects due to the proximity and relative position of two bubbles in a sound field

Kapodistrias, G., and P.H. Dahl, "On scattering effects due to the proximity and relative position of two bubbles in a sound field," J. Acoust. Soc. Am., 108, 2485, 2000.

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1 Nov 2008

Recently the authors published results from a theoretical and experimental investigation on scattering of sound from two bubbles symmetrically arranged about the combined beam axis of a set of transducers [J. Acoust. Soc. Am. 107, 3006–3017 (2000)]. In this presentation the investigation is extended to two additional geometries, with the two bubble array placed at angles of 0° and 45° from the combined beam axis. For each angle, the half interbubble distance d is varied such that the dimensionless variable kd ranges from 0.2–5 (for the 0° case) and 0.2–10 (for the 45° case), where k is the acoustic wave number. Modeling is accomplished by using a closed-form solution derived from the multiple scattering series, with the bubble scattering function expressed in terms of spherical harmonics. Experimental data are obtained by symmetrically arranging two bubbles, each of radius a=425 µm, on a fine nylon thread, with the bubbles insonified by tone bursts with a center frequency of 120 kHz. The data closely agree with the simulations, and it is verified that, regardless of the geometry, for kd≤1 the response of the two bubble array drastically departs from the one due to single scattering. This departure is attributed to multiple scattering and is manifested as a reduction in backscattered radiation.

Field measurements and modeling of attenuation from near-surface bubbles for frequencies 1-20 kHz

Dahl, P.H., J.W. Choi, N.J. Williams, and H.C. Graber, "Field measurements and modeling of attenuation from near-surface bubbles for frequencies 1-20 kHz," J. Acoust. Soc. Am., 124, EL163-EL169, 2008.

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28 Aug 2008

Measurements of excess attenuation from near-surface bubbles from the Shallow Water '06 experiment are reported. These are transmission measurements made over the frequency range 1–20 kHz, and they demonstrate a frequency, grazing angle, and wind speed dependence in attenuation. Data modeling points to bubble void fractions of order 10-6 in effect for wind speeds 10–13 m/s. Simultaneous measures of wind speed made within 1.5 and 11 km of the open water experimental location differed by 2 m/s in their respective 30-min average; this has cautionary implications for empirical models for bubble attenuation that are a strong function of wind speed.

Mid-frequency acoustic propagation in shallow water on the New Jersey shelf. II: Intensity fluctuation

Tang, D., F.S. Henyey, Z. Wang, K.L. Williams, D. Rouseff, P.H. Dahl, J. Quijano, and J.W. Choi, "Mid-frequency acoustic propagation in shallow water on the New Jersey shelf. II: Intensity fluctuation," J. Acoust. Soc. Am., 124, EL91-EL96, 2008.

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28 Aug 2008

The scintillation index and the intensity cumulative distribution function of mid-frequency (2–10 kHz) sound propagation are presented at ranges of 1–9 km in a shallow water channel. The fluctuations are due to water column sound speed variability. It is found that intensity is only correlated over a narrow frequency band (50–200 Hz) and the bandwidth is independent of center frequency and range. Furthermore, the intensity probability distribution peaks at zero for all frequencies, and follows an exponential distribution at small values.

Mid-frequency acoustic propagation in shallow water on the New Jersey shelf: Mean intensity

Tang, D., F.S. Henyey, Z. Wang, K.L. Williams, D. Rouseff, P.H. Dahl, J. Quijano, and J.W. Choi, "Mid-frequency acoustic propagation in shallow water on the New Jersey shelf: Mean intensity," J. Acoust. Soc. Am., 124, EL85-EL90, 2008.

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28 Aug 2008

Mid-frequency (1–10 kHz) sound propagation was measured at ranges 1–9 km in shallow water in order to investigate intensity statistics. Warm water near the bottom results in a sound speed minimum. Environmental measurements include sediment sound speed and water sound speed and density from a towed conductivity-temperature-depth chain. Ambient internal waves contribute to acoustic fluctuations. A simple model involving modes with random phases predicts the mean transmission loss to within a few dB. Quantitative ray theory fails due to near axial focusing. Fluctuations of the intensity field are dominated by water column variability.

Observations of the R reflector and sediment interface reflection at the Shallow Water '06 central site

Choi, J.W., P.H. Dahl, and J.A. Goff, "Observations of the R reflector and sediment interface reflection at the Shallow Water '06 central site," J. Acoust. Soc. Am., 124, EL128-EL134, 2008.

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28 Aug 2008

Acoustic bottom-interacting measurements from the Shallow Water '06 experiment (frequency range 1–20 kHz) are presented. These are co-located with coring and stratigraphic studies showing a thin (~ 20 cm) higher sound speed layer overlaying a thicker ( ~ 20 m) lower sound speed layer ending at a high-impedance reflector (R reflector). Reflections from the R reflector and analysis of the bottom reflection coefficient magnitude for the upper two sediment layers confirm both these features. Geoacoustic parameters are estimated, dispersion effects addressed, and forward modeling using the parabolic wave equation undertaken. The reflection coefficient measurements suggest a nonlinear attenuation law for the thin layer of sandy sediments.

Fluctuation of mid-frequency propagation in shallow water

Tang, D., F.S. Henyey, K.L. Williams, D. Rouseff, P.H. Dahl, Z. Wang, J.E. Quijano, and J.W. Choi, "Fluctuation of mid-frequency propagation in shallow water," J. Acoust. Soc. Am., 123, 3433, 2008.

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1 May 2008

As part of the ONR-sponsored SW06 experiment, mid-frequency sound propagation was measured at ranges 1–10 km in the frequency band of 2–10 kHz in August, 2006. The water depth is 80 m and the source depth is 30 m, close to the minimum of a duct with a thermocline above and a warm salty water below. The receivers are clustered into two groups, one at 25 m depth, the other at 50 m. The region has active internal wave activity during this time. Because the source is near the axis of the sound channel, it is observed that propagation is dominated by trapped modes and behaves similar to sound propagation in a deep water duct. Amplitude fluctuations and cross-frequency correlations are estimated. The scintillation index as a function of frequency and bandwidth is calculated.

Mid-frequency bottom interaction measurements off the New Jersey continental shelf

Choi, J.W., and P.H. Dahl, "Mid-frequency bottom interaction measurements off the New Jersey continental shelf," J. Acoust. Soc. Am., 123, 3084, 2008.

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1 May 2008

Two kinds of bottom interaction measurements conducted in waters 80 m deep off the North American continental shelf as part of Shallow Water 06 (August, 2006) are discussed. In each, acoustic signals were recorded on two, colocated vertical line arrays of length 1.4 m, one at depth 25 m, and other at depth 50 m. The source was deployed at depth 40 m from the R/V Knorr that could either be positioned or towed at rate 0.1 m/s. The first, "bottom reflection" is interpreted as a measure of the modulus of the plane wave reflection coefficient as functions of frequency (1–20 kHz) and grazing angle associated with the discrete set of ranges (50–300 m). The second represents a "move-out" type measurement with source towed away from the receiver, and reflection at continuous angles associated with the 50–300 m range. Frequency range was 1–2 kHz, and as the source was 5 m off the bottom, spherical wave effects are investigated. Both measurements were carried out over the same four directions originating from the receiver, each separated by 90 degrees. Physical processes responsible for the observations in each case are discussed and modeled.

Shallow Water '06: A joint acoustic propagation/nonlinear internal wave physics experiment

Tang, D., J.F. Moum, J.F. Lynch, P. Abbott, R. Chapman, P.H. Dahl, T.F. Duda, G. Gawarkiewicz, S. Glenn, J.A. Goff, H. Graber, J. Kemp, A. Maffei, J.D. Nash, and A. Newhall, "Shallow Water '06: A joint acoustic propagation/nonlinear internal wave physics experiment," Oceanography, 20, 156-167, 2007.

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1 Dec 2007

Since the end of the Cold War, the US Navy has had an increasing interest in continental shelves and slopes as operational areas. To work in such areas requires a good understanding of ocean acoustics, coastal physical oceanography, and, in the modern era, autonomous underwater vehicle (AUV) operations.

Environmental noise studies in Puget Sound

Dall'Osto, D., and P.H. Dahl, "Environmental noise studies in Puget Sound," J. Acoust. Soc. Am., 122, 3083, 2007.

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1 Nov 2007

The ambient noise environment at a site in North Puget Sound, Washington, depth ~100 m, located within the nearby Smith Island marine sanctuary, is studied. The measurement system consists of a buoy for which the surface expression houses a microphone at nominal height 2 m above the sea surface, with multiple underwater acoustic sensors suspended in the water column. The recording bandwidth for the air system corresponds to audio band, whereas the underwater system records ambient noise at frequencies up to 50 kHz. The two systems are recorded coherently. One goal of this pilot study is to examine different components of the noise budget, including injection of noise from airplane flyovers, and correlation between pressures above and below water. Another goal relates to properties of the noise field that could possibly impact echolocation by southern resident killer whales; these properties likely being restricted to noise levels at frequencies above 25 kHz. Besides representing an important marine mammal habitat, a key advantage of this site is the availability of meteorological and sea surface wave data, obtained from Smith Island and a nearby NOAA buoy, and vessel and air traffic data. Results from the summer 2007 field work will be presented.

High-frequency underwater sound

Dahl, P.H., "High-frequency underwater sound," In: Encyclopedia of Electrical and Electronics Engineering, edited by J.G. Webster, 1-19 (New York: John Wiley & Sons, 2007).

30 Jan 2007

Underwater ambient noise

Dahl, P.H., J.H. Miller, D.H. Cato, and R.K. Andrew, "Underwater ambient noise," Acoustics Today, 1, 23-33, 2007.

30 Jan 2007

Precursor arrivals in the Yellow Sea, their distinction from first-order head waves, and their geoacoustic inversion

Dahl, P.H., and J.W. Choi, "Precursor arrivals in the Yellow Sea, their distinction from first-order head waves, and their geoacoustic inversion," J. Acoust. Soc. Am., 120, 3525-3533, 2006.

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1 Dec 2006

Measurements made as part of the 1996 Yellow Sea experiment at location 37°N, 124°E, undertaken by China and the U.S. are analyzed. Signals generated by explosive sources were received by a 60-m-length vertical line array deployed in waters 75 m deep. Evidence is presented that precursor arrivals measured at ranges less than 1 km are refracted waves that are zeroth order in their ray series classification, and this directly points to the existence of a gradient in sediment sound speed. In contrast, first-order head waves, which are much weaker in amplitude, would exist only if this gradient were absent. It is found that the energy spectrum of precursor arrivals agrees well with a zeroth-order model, i.e., it is proportional to the source amplitude spectrum, S(f), where f is frequency, rather than a first-order model, which would have it proportional to S(f)/f. From travel time analysis the sediment sound speed just below the water-sediment interface is estimated to be 1573 m/s with a gradient of 1.1 s-1, and from analysis of the energy spectrum of the precursor arrivals the sediment attenuation is estimated to be 0.08 dB/m/kHz over the frequency range 150–420 Hz. The results apply to a nominal sediment depth of 100 m.

First-order and zeroth-order head waves, their sequence, and implications for geoacoustic inversion

Choi, J.W., and P.H. Dahl, "First-order and zeroth-order head waves, their sequence, and implications for geoacoustic inversion," J. Acoust. Soc. Am., 119, 3660-3668, doi:10.1121/1.2195110, 2006.

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1 Jun 2006

The relation between the head wave and an arrival often called a ground wave is analyzed with parabolic equation-based simulations, and an interpretation of such a ground wave as a head wave sequence is presented. For a Pekeris waveguide the envelope of the spectrum of the ground wave arrival corresponds to the spectrum of a single head wave, and spectral peaks correspond to odd multiples of the mode-1 cutoff frequency. This head wave is first order in its ray series classification and its amplitude spectrum goes as|S(f)|/f, where S(f) is the source amplitude spectrum. Basic variations from a Pekeris waveguide are considered; isospeed layers or a positive sound speed gradient in the seabed can each give rise to arrivals that are zeroth order in ray series classification and higher amplitude. For a sound speed gradient there is either a low-amplitude interference head wave whose properties are akin to a first-order head wave, or a high-amplitude interference head wave or non-interfering refracted wave whose properties are zeroth order and spectra follow |S(f)|. Parametric dependences for distinguishing these arrivals and implications for geoacoustic inversion are discussed.

Geoacoustic inversion in the Yellow Sea using precursor arrivals

Dahl, P.H., and J.W. Choi, "Geoacoustic inversion in the Yellow Sea using precursor arrivals," Proceedings, Ninth Western Pacific Acoustics Conference, Seoul, South Korea, June 2006.

1 Jun 2006

Measurement and simulation of the channel intensity impulse response for a site in the East China Sea

Choi, J.W., and P.H. Dahl, "Measurement and simulation of the channel intensity impulse response for a site in the East China Sea," J. Acoust. Soc. Am., 119, 2677-2685, doi:10.1121/1.2189449, 2006.

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1 May 2006

A model for the channel intensity impulse response Ic(t) is presented that is generally applicable for source-receiver ranges less than ten water depths. The separate impulse response functions from each arrival, such as the single surface bounce or surface-to-bottom bounce, are modeled using bistatic scattering concepts and are incoherently summed for the total response function. The expression Ic(t) is equivalent to a time-averaged response and embodies the boundary scattering and reflection physics corresponding to the center frequency at which computations are made. To compare with observations, Ic(t) is convolved with representations of the 8- and 16-kHz continuous wave (CW) pulses, and an 8–16-kHz frequency-modulated (FM) pulse, that were used in the Asian Sea International Acoustics Experiment conducted in the East China Sea (depth 105 m). For the FM case the computation frequency is 12 kHz, the center frequency of the FM pulse. It is found that six primary arrivals dominate the response for ranges less than about 1 km. With modeling of Ic(t) limited to these paths, the basic structure of Ic(t) is set by bottom properties and acquisition geometry with some changes in intrapath time spreading that depend on sea surface conditions.

The East China Sea as an underwater acoustic communication channel: Measurements of the channel impulse response

Dahl, P.H., and J.W. Choi, "The East China Sea as an underwater acoustic communication channel: Measurements of the channel impulse response," U.S. Navy J. Underwater Acoust., 56, 141-154, 2006.

15 Jan 2006

Observations of zeroth-order head waves in the Yellow Sea

Dahl, P.H., and J.W. Choi, "Observations of zeroth-order head waves in the Yellow Sea," J. Acoust. Soc. Am., 118, 1969, 2005

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1 Sep 2005

As part of the 1996 joint U.S–China Experiment in the Yellow Sea, head wave arrivals were measured in waters 75 m deep and at range ~700 m, on a vertical line array (VLA) that spanned the water column. Head waves are strongly linked with properties of the seabed, and thus provide a useful measure for geoacoustic inversion carried out at relatively short range. Based on a time-domain and beam-forming analysis of the VLA data, the sediment sound speed and its gradient are estimated, and the conclusion is made that the observed head waves are kinematically consistent with zeroth-order in their ray-series classification. This conclusion is verified by simulation using the RAM parabolic equation algorithm, and it is found that the dynamic properties of the data also point to a zeroth-order, rather than a first-order, classification. The classification in turn establishes a baseline model for the head wave energy spectrum that is compared with the measured data. It is found that the estimated energy spectrum agrees well with a zero-order model, i.e., proportional to the source spectrum, S(f), modified by sediment attenuation, rather than a first-order model, S(f)/f.

On spectral and amplitude properties of first- and zero-order head waves, and implications for geoacoustic inversion

Choi, J.W., and P.H. Dahl, "On spectral and amplitude properties of first- and zero-order head waves, and implications for geoacoustic inversion," J. Acoust. Soc. Am., 118, 1969, 2005.

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1 Jul 2005

Acoustic cues related to the voice source, including harmonic structure and spectral tilt, were examined for relevance to prosodic boundary detection. The measurements considered here comprise five categories: duration, pitch, harmonic structure, spectral tilt, and amplitude. Distributions of the measurements and statistical analysis show that the measurements may be used to differentiate between prosodic categories. Detection experiments on the Boston University Radio Speech Corpus show equal error detection rates around 70% for accent and boundary detection, using only the acoustic measurements described, without any lexical or syntactic information. Further investigation of the detection results shows that duration and amplitude measurements, and, to a lesser degree, pitch measurements, are useful for detecting accents, while all voice source measurements except pitch measurements are useful for boundary detection.

Alaskan river environmental acoustics

Dahl, P.H., C. Pfisterer, and H.J. Geiger, "Alaskan river environmental acoustics," J. Acoust. Soc. Am., 117, 2380, 2005

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2 Apr 2005

Sonars are used by the Alaska Department of Fish and Game (ADF&G) to obtain daily and hourly estimates of at least four species of migratory salmon during their seasonal migration which lasts from June to beginning of September. Suspended sediments associated with a river's sediment load is an important issue for ADF&G's sonar operations. Acoustically, the suspended sediments are a source of both volume reverberation and excess attenuation beyond that expected in fresh water. Each can impact daily protocols for fish enumeration via sonar. In this talk, results from an environmental acoustic study conducted in the Kenai River (June 1999) using 420 kHz and 200 kHz side looking sonars, and in the Yukon River (July 2001) using a 120 kHz side looking sonar, are discussed. Estimates of the volume scattering coefficient and attenuation are related to total suspended sediments. The relative impact of bubble scattering and sediment scattering is also discussed.

Forward scattering from the sea surface: Observations of both subtle and profound effects of bubbles in single-interaction measurements

Dahl, P.H., "Forward scattering from the sea surface: Observations of both subtle and profound effects of bubbles in single-interaction measurements," J. Acoust. Soc. Am., 117, 2433, 2005.

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1 Apr 2005

For frequencies of O(10) kHz and above, field data show that near-surface bubbles impact forward scattering from the sea surface in three phases. The first occurs under mild conditions (wind speed less than 5–7 m/s); here a pulse forward scattered from the sea surface is extended in time, but only at levels ~30 dB below the peak level, which itself is not attenuated. The second occurs under more vigorous conditions (wind speed 7–12 m/s); here a significant energy loss is observed, but time and angle spreading (dominated by rough surface scattering) remain relatively unchanged. The third occurs under still more vigorous conditions (wind speed greater than ~12 m/s). Here, there is near total occlusion of the sea surface, time and angle spreading are manifestly altered, and bubble-mediated energy loss becomes bounded by scattering from bubbles. Examples from ASIAEX East China Sea and other archival data sets will be discussed along with a model for bubble-mediated energy loss in forward scattering from the sea surface. In the case of near total occlusion, an interesting example of the knock-down of horizontal coherence will be discussed along with a model that utilizes the van Cittert–Zernike Theorem.

Basic technologies in ultrasound

Matre, K. and P.H. Dahl, "Basic technologies in ultrasound," in Basic and New Aspects in Gastrointestinal Ultrasoundography, S. Odegaard, H. Gregerson, and O. Giija, eds. (World Scientific, Amsterdam, 2005)

30 Jan 2005

The sea surface bounce channel: Bubble-mediated energy loss and time/angle spreading

Dahl, P.H., "The sea surface bounce channel: Bubble-mediated energy loss and time/angle spreading," Proceedings, American Institute of Physics Conference on High Frequency Acoustics, edited by M.B. Porter, M. Siderius, and W.K. Kuperman, vol 728, pp. 194-204, doi:10.1063/1.1843013 (New York, 2004).

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18 Nov 2004

A model is presented for the energy loss in the sea surface forward bounce channel due to attenuation from wind-speed-dependent bubbles; the model is compared to data from ASIAEX and other archival data sets. At high wind speeds the model predicts an energy loss bound, i.e., no further attenuation with increasing wind speed. Prior to reaching this bound and while there is attenuation, time and angle spreading in the forward bounce path remain largely controlled by the spectral properties of the air–sea interface, i.e., they remain unchanged by the bubbles. Once bounding of energy loss occurs, initiated by the dominance of bubble scattering over air–sea interface scattering, time and angle spreading of the arrival change profoundly.

Bound and free surface waves in a large wind-wave tank

Plant, W.J., P.H. Dahl, J.-P. Giovanangeli, and H. Branger, "Bound and free surface waves in a large wind-wave tank," J. Geophys. Res., 109, 10.1029/2004JC002342, 2004.

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5 Oct 2004

Microwave and acoustic systems operated in the large wind-wave tank in Luminy, Marseille, France, show that most small-scale waves produced at large angles to the wind are products of breaking events bound to longer waves in the tank. These longer waves propagate at the dominant wave phase speed for fetches near 7 m but travel at speeds corresponding to the phase speed of a wave half as long as the dominant wave at fetches near 26 m. The microwave and acoustic systems operated at both 8 mm and 2 cm wavelengths. They were set to look at the same surface spot simultaneously at the same incidence and azimuth angles. Measurements were made at seven wind speeds, five incidence angles, seven azimuth angles, and two nominal fetches. Two peaks were found in either the microwave or acoustic Doppler spectrum when looking upwind or downwind but never in both. The low-frequency peak is due to Bragg scattering from freely propagating short waves, while the high-frequency peak is a result of Bragg scattering from short waves bound to longer waves. At azimuth angles not aligned with the wind direction the high-frequency peak was found to move lower until it merged with the low-frequency peak at azimuth angles around 60°.

Fitting the first moments of these Doppler spectra along with the backscattering cross sections to a model of free wave/bound wave scattering showed that the intensity of bound and free short waves generally decreased with azimuth angle but that free wave spectral densities decreased more rapidly. Differences in microwave and acoustic cross sections confirmed that the bound waves were tilted by their parent waves. Spectral densities of bound and free waves were estimated individually by fitting the data to the model. The sum of these spectral densities, the total short-wave spectral density, was similar to, but lower than, previous measurements. The nature of millimeter-length bound waves was found to be different at long fetches than at short fetches, a feature not observed in centimeter-length bound waves.

Geoacoustic inversion results from the ASIAEX East China Sea Experiment

Potty, G.R., J.H. Miller, P.H. Dahl, and C.J. Lazauski, "Geoacoustic inversion results from the ASIAEX East China Sea Experiment," IEEE J. Ocean. Eng., 29, 1000-1011, doi:10.1109/JOE.2004.833224, 2004.

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1 Oct 2004

This work presents the results of geoacoustic inversions carried out using data from the Asian Seas International Acoustics Experiment East China Sea. Broadband data from small explosive sources were used for the inversions. Compressional wave speeds in the sediment and basement layers were estimated using a nonlinear, long-range, tomographic inversion technique based on group speed dispersion. This tomographic technique is a hybrid approach that combines a genetic algorithm for global parameter search with a Levenberg-Marquardt method for fine-scale parameter tuning. The results were compared with data from gravity and piston cores and a geophysical survey conducted at the experimental location using a watergun and towed hydrophone array.

Mid to high frequency bottom loss in the East China Sea

Choi, J.W., and P.H. Dahl, "Mid to high frequency bottom loss in the East China Sea," IEEE J. Ocean. Eng., 29, 1038-1045, doi:10.1109/JOE.2004.834178, 2004.

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1 Oct 2004

Bottom-loss measurements made in the East China Sea in May–June 2001 as part of the Asian Sea International Acoustics Experiment as a function of frequency (2–20 kHz) and seabed grazing angle (15°&3150;24°) are presented. The measurements are interpreted as estimates of the modulus of the plane wave reflection coefficient and data are compared to predicted values using a reflection coefficient model, based on a two-layered sediment for which the sound speed in the surficial sediment layer is allowed to vary as a linear k2 profile, where k is acoustic wave number. The region below this layer is modeled as a half-space with constant density and sound speed. The reflection coefficient model is driven by eight geoacoustic parameters; these are estimated from the data by minimizing the weighted squared error between the data and the model predictions for a candidate set of parameters. The parameter estimates for the sediment layer are thickness, 0.9±0.5 m; density, 2.0±0.1 g/cm3; and attenuation, 0.25±0.05 dB/m/kHz, with sediment layer sound speed increasing from 1557±4 m/s at the water-sediment interface to 1625±35 m/s at a depth of 0.9 m. The parameter estimates for the half-space are density, 2.0±0.1 g/cm3; attenuation, 0.25±0.15 dB/m/kHz; and sound speed, 1635±52 m/s. Variances for these estimates are derived using the Bootstrap method. This parameter set produced model curves that agreed reasonably well with the observations of bottom loss over the entire frequency range and is consistent with the range of independently measured geoacoustic variables. Since this mid-to-high-frequency data set does not provide detailed information about the sediment structure for depths beyond about 3 m, the geoacoustic parameter set is more properly viewed as description of the sediment layer and sediments in the underlying 2 m. Similarly, a self-consistent construction of a geoacoustic model for the East China Sea should necessarily amalgamate the mid-to-high-frequency results given here with results obtained at lower frequencies.

Overview of the results from the Asian Seas International Acoustics Experiment in the East China Sea

Dahl, P.H., R. Zhang, J.H. Miller, L.R. Bartek, Z. Peng, S.R. Ramp, J.-X. Zhou, C.S. Chiu, J.F. Lynch, J.A. Simmen, and R.C. Spindel, "Overview of the results from the Asian Seas International Acoustics Experiment in the East China Sea," IEEE J. Ocean. Eng., 29, 920-928, doi:10.1109/JOE.2005.843159, 2004.

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1 Oct 2004

The Asian Seas International Acoustics Experiment (ASIAEX) included two major field programs, one in the South China Sea and the other in the East China Sea (ECS). This paper presents an overview of research results from ASIAEX ECS conducted between May 28 and June 9, 2001. The primary emphasis of the field program was shallow-water acoustic propagation, focused on boundary interaction and geoacoustic inversion. The study area's central point was located at 29° 40.67'N, 126° 49.39'E, which is situated 500 km east of the Chinese coastline off Shanghai. The acoustic and supporting environmental measurements are summarized, along with research results to date, and references to papers addressing specific issues in more detail are given.

Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water

Zhou, J.-X., X.-Z. Zhang, P.H. Rogers, J.A. Simmen, P.H. Dahl, G. Jin, and Z. Peng, "Reverberation vertical coherence and sea-bottom geoacoustic inversion in shallow water," IEEE J. Ocean. Eng., 29, 988-999, doi:10.1109/JOE.2004.836401, 2004.

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1 Oct 2004

Optimal array-processing techniques in the ocean often require knowledge of the spatial coherence of the reverberation. A mathematical model is derived for the reverberation vertical coherence (RVC) in shallow water (SW). A method for analysis of RVC data is introduced. Measured reverberation cross-correlation coefficients as a function of time and frequency, obtained during the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, are reported. SW reverberation from a single shot provides a continuous spatial sampling of the surrounding sound field up to several tens of kilometers and holds valuable information on the geoacoustic properties of the sea floor over this distance. SW reverberation data can, therefore, be used as the basis for a quick and inexpensive method for geoacoustic inversion and has the obvious advantage that acquiring the data in situ requires only a single platform. This paper considers the use of the vertical coherence of the reverberation as the starting point for such an inversion. Sound speed and attenuation in the sea bottom at the ASIAEX site are obtained over a frequency range of 100-1500 Hz by finding values that provide the best match between the measured and predicted RVC.

Seabed acoustic parameters from dispersion analysis and transmission loss in the East China Sea

Peng, Z., J.-X. Zhou, P.H. Dahl, and R. Zhang, "Seabed acoustic parameters from dispersion analysis and transmission loss in the East China Sea," IEEE J. Ocean. Eng., 29, 1038-1045, doi:10.1109/JOE.2004.836400, 2004.

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1 Oct 2004

As a part of the Asian Seas International Acoustic Experiment (ASIAEX) in the East China Sea, sound propagation signals from wideband explosive sources were measured using a 32-element vertical line array. Measurements were made as a function of range in two perpendicular tracks. Sea-bed geoacoustic parameters based on a fluid half-space geoacoustic model (sound speed, density, and attenuation) are inverted from the sound propagation in the frequency range 100–500 Hz. The sea-bed sound speed and density were first derived from mode arrival time differences which were obtained using a spatial mode filtering technique. Sea-bed acoustic attenuation was subsequently estimated by comparing measured transmission loss with model results.

The shelf-edge environment in the central East China Sea and its impact on low-frequency acoustic propagation

Ramp, S.R., C.S. Chiu, F.H. Bahr, Y. Qi, P.H. Dahl, J.H. Miller, J.F. Lynch, R. Zhang, and J.-X. Zhou, "The shelf-edge environment in the central East China Sea and its impact on low-frequency acoustic propagation," IEEE J. Ocean. Eng., 29, 1011-1031, doi:10.1109/JOE.2004.840842, 2004.

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1 Oct 2004

Two field programs, both parts of the Asian Seas International Acoustics Experiment (ASIAEX), were carried out in the central East China Sea (28° to 30° N, 126° 30' to 128°E) during April 2000 and June 2001. The goal of these programs was to study the interactions between the shelf edge environment and acoustic propagation at a wide range of frequencies and spatial scales. The low-frequency across-slope propagation was studied using a synthesis of data collected during both years including conductivity-temperature-depth (CTD) and mooring data from 2000, and XBT, thermistor chain, and wide-band source data from 2001. The water column variability during both years was dominated by the Kuroshio Current flowing from southwest to northeast over the continental slope. The barotropic tide was a mixed diurnal/semidiurnal tide with moderate amplitude compared to other parts of the Yellow and East China Sea. A large amplitude semidiurnal internal tide was also a prominent feature of the data during both years. Bursts of high-frequency internal waves were often observed, but these took the form of internal solitons only once, when a rapid off-shelf excursion of the Kuroshio coincided with the ebbing tide.

Two case studies in the acoustic transmission loss (TL) over the continental shelf and slope were performed. First, anchor station data obtained during 2000 were used to study how a Kuroshio warm filament on the shelf induced variance in the transmission loss (TL) along the seafloor in the NW quadrant of the study region. The corresponding modeled single-frequency TL structure explained the significant fine-scale variability in time primarily by the changes in the multipath/multimode interference pattern. The interference was quite sensitive to small changes in the phase differences between individual paths/modes induced by the evolution of the warm filament. Second, the across-slope sound speed sections from 2001 were used to explain the observed phenomenon of abrupt signal attenuation as the transmission range lengthened seaward across the continental shelf and slope. This abrupt signal degradation was caused by the Kuroshio frontal gradients that produced an increasingly downward-refracting sound-speed field seaward from the shelf break. This abrupt signal dropout was explained using normal mode theory and was predictable and source depth dependent. For a source located above the turning depth of the highest-order shelf-trapped mode, none of the propagating modes on the shelf were excited, causing total signal extinction on the shelf.

Forward scattering from the sea surface and the van Cittert-Zernike theorem

Dahl, P.H., "Forward scattering from the sea surface and the van Cittert-Zernike theorem," J. Acoust. Soc. Am., 115, 589-599, 2004.

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1 Feb 2004

The van Cittert–Zernike theorem is used to generate models for the spatial coherence of a sound field that has been forward scattered from the sea surface. The theorem relates the spatial coherence of an observed wave field to the distant source intensity distribution associated with this field. In this case, the sea surface upon ensonification is taken to be the source, and the sea-surface bistatic cross section corrected for transmission loss is taken as a surrogate for the source intensity distribution. Improvements in methodology for generating an estimate of the 2D autocorrelation function for sea surface waveheight variation, necessary to compute the bistatic cross section, are documented in the Appendix. Upon invoking certain approximations, simple expressions for the characteristic length scales of vertical, horizontal, and horizontal–longitudinal coherence, are derived from the theorem. The three coherence length scales identify a coherence volume for the spatial coherence of a sound field arriving via the surface bounce channel. Models for spatial coherence derived from the van Cittert–Zernike theorem without these approximations compare reasonably well with measurements of complex vertical coherence made at 8 kHz and 20 kHz in the East China Sea as part of the 2001 ASIAEX field program. In terms of the ASIAEX field geometries and sea-surface conditions, at frequency of 20 kHz the coherence volume is a vertical layer 0.5 m thick by 3 m in each of the two horizontal dimensions; at 8 kHz these dimensions increase by a factor of 2.5, representing the ratio of the two frequencies.

Acoustic tomography for monitoring the Sea of Japan: a pilot experiment

Spindel, R.C., J. Na, P.H. Dahl, S. Oh, C. Eggen, Y.G. Kim, V.A. Akulichev, and Y.N. Morgunov, "Acoustic tomography for monitoring the Sea of Japan: a pilot experiment," IEEE J. Ocean. Eng., 28, 297-302, 10.1109/JOE.2003.811896, 2003.

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9 Jul 2003

A pilot experiment was conducted in the Sea of Japan (also called the East Sea) in September-October 1999, to assess the possibility of using acoustic tomographic techniques for monitoring water mass structure and dynamics. Acoustic m-sequence signals at various frequencies between 250 and 634 Hz were transmitted from bottom-mounted acoustic sources in shallow water off the coast of Vladivostok to vertical-array receiving systems deployed off the north coast of Ulleung-Do island (S. Korea), 558 km to the south. The data are analyzed for temporal correlation, time spread, and transmission loss and are interpreted in terms of a tomographic system for monitoring the East Sea.

Program fosters advances in shallow-water acoustics in southeastern Asia

Ramp, S.R., J.F. Lynch, P.H. Dahl, C.-S. Chiu, and J.A. Simmen, "Program fosters advances in shallow-water acoustics in southeastern Asia," Eos Trans. AGU, 84, 361 and 367, doi:10.1029/2003EO370001, 2003.

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30 Jan 2003

Two impressive field programs were recently completed in the South and East China Seas as part of the Asian Seas International Acoustics Experiment (ASIAEX). Under the direction of the U.S. Office of Naval Research (ONR), scientists from several countries joined together to collect unique and high-quality acoustical, oceanographic, and geophysical data at the same place and time, enabling underwater acoustic fluctuations to be understood and modeled at the space and time scales of interest. A comprehensive field effort such as this one is required to decipher the cause and effect between environmental and acoustic variability which exist on a wide range of time and space scales.

Groundwork for the two largest experiments of ASIAEX was laid with preliminary survey and studies of spring in 2000. In 2000 in the East China Sea (ECS), a 21-day cruise on the R/V Revelle allowed extensive geophysical surveys of the upper sea floor using chirp sonars and a low-frequency air gun, as well as allowing some sampling of the physical oceanography. In the same year in the South China Sea (SCS), a SeaSoar survey aboard the R/V Ocean Researcher 1 delineated the major meso-scale features of the northeastern SCS, while a limited moored array obtained a first look at the dramatic non-linear internal waves. These pilot field studies proved indispensable when designing the two main field programs that were executed in spring 2001.

Scattering from a single bubble near a roughened air-water interface: Laboratory measurements and modeling

Dahl, P.H., and G. Kapodistrias, "Scattering from a single bubble near a roughened air-water interface: Laboratory measurements and modeling," J. Acoust. Soc. Am., 113, 94-101, 2003.

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30 Jan 2003

The problem of scattering from a single bubble located close to a slightly roughened, air–water interface is studied both theoretically and experimentally. Two well-controlled laboratory experiments were performed to investigate the effects of surface roughness on the scattering response of the bubble. In the first experiment, a bubble of radius 1200 μm was placed on a fine thread at a variable distance, d, from the mean-still-water level of the surface, which was roughened using a wind source. In the second experiment, a bubble of radius 800 μm was utilized, while the water surface was roughened using a plunger wave-making source. The waveheights and important characteristic length scales associated with each experiment were quantified using digital photography. The wind source produced waveheights that were represented by a Gaussian distribution, while the plunger source produced waveheights that were represented by a bimodal distribution. To model the acoustic measurements, an expression describing the four scattering paths, from source to bubble to receiver, was used. A random phase shift due to the surface roughness was added to the paths that interacted with the surface, and expectations of this phase shift were computed based on the analytical representations for the waveheight distribution. The data show good agreement with the simulations and the sensitivity of scattering from a subsurface bubble to small changes in waveheight is illustrated. The experiments highlight important parametric dependencies, which are summarized here, and the relation between monostatic and bistatic scattering is also discussed.

The contribution of bubbles to high-frequency sea surface backscatter: A 24-h time series of field measurements

Dahl, P.H., "The contribution of bubbles to high-frequency sea surface backscatter: A 24-h time series of field measurements," J. Acoust. Soc. Am., 113, 769-780, doi:10.1121/1.1532029, 2003.

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30 Jan 2003

Measurements of acoustic sea surface backscattering, wind speed, and surface wave spectra were made continually over a 24-h period in an experiment conducted in 26 m of water near the Dry Tortugus collection of islands off south Florida in February 1995. The backscattering measurements were made at a frequency of 30 kHz and a sea surface grazing angle of 20°; a time series of the decibel equivalent of this variable, called SS20, was studied in terms of its dependence on environmental variables. On occasion reliable estimates of scattering in the grazing range 15°–27° were also obtained during the 24 hours. The scattering data exhibited evidence, in terms of scattering level and grazing angle dependence, of scattering from near-surface bubbles rather than scattering from the rough air–sea interface. The scattering data were compared with a model for σb, the apparent backscattering cross section per unit area due to bubble scattering, that is driven by a parameter, βI, equal to the depth-integrated extinction cross section per unit volume. Using an empirical model for βI based on data from a 1977 experiment conducted in pelagic waters, model predictions agreed reasonably well with the 1995 measurements presented here. Additional model–data comparisons were made using four measurements from a 1992 experiment conducted in pelagic waters. Finally, the 24-h time series of acoustic scattering exhibited a hysteresis effect, wherein for a given wind speed, there was a tendency for the scattering level to be higher if prior winds had been falling. A better understanding of this effect is essential to reduce uncertainty in model predictions.

Proceedings, The Asian Seas International Acoustics Experiment (ASIAEX) International Symposium, Chengdu, China, October 14-18, 2002

Tang, D., S.R. Ramp, P.H. Dahl, J. Lynch, J. Zhou, R. Zhang, C.-S. Chiu, R.C. Spindel, R.C., and J. Simmen, "Proceedings, The Asian Seas International Acoustics Experiment (ASIAEX) International Symposium, Chengdu, China, October 14-18, 2002," APL-UW TR 0201, December 2002.

30 Dec 2002

On scattering from a bubble located near a flat air–water interface: Laboratory measurements and modeling

Kapodistrias, G., and P.H. Dahl, "On scattering from a bubble located near a flat air–water interface: Laboratory measurements and modeling," J. Acoust. Soc. Am., 110, 1271-1281, 2001.

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1 Sep 2001

Scattering by a single bubble near a flat air–water interface is investigated theoretically and experimentally. A ray-acoustic interpretation is used to describe the four scattering paths, from source to bubble to receiver, that determine the response of the bubble. Multiple scattering effects are accounted for using a closed-form solution derived from the multiple scattering series. Experiments are performed by placing a bubble with radius a ≈ 425 µm on a fine nylon thread, which is approximately 100 µm in diameter and practically transparent to sound, at a distance d from the interface. The primary variable is d and it ranges from 1a to 100a. The bubble is excited by tone bursts with a center frequency of 120 kHz, with the transducers arranged in both bistatic and monostatic configurations. Theory and experiment are in good agreement, verifying the dominant effect of the four paths in the response of the bubble, with multiple scattering playing a role for kd<1, where k is the wave number of the medium. In the long-range limit our simulations agree with those of Ye and Feuillade [J. Acoust. Soc. Am. 102, 798-805 (1997)] including the shifting of the bubble's resonant frequency. The dependence of scattering on transducer arrangement, range to bubble, grazing angle, and phase relation among the four paths, vis-à-vis monostatic and bistatic scattering, is discussed.

ASIAEX, East China Sea: Cruise Report of the Activities of the R/V Melville 29 May to 9 June 2001

Dahl, P.H., "ASIAEX, East China Sea: Cruise Report of the Activities of the R/V Melville 29 May to 9 June 2001," APL-UW TM 7-01, July 2001.

1 Jul 2001

Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars

Dahl, P.H., "Bubble clouds and their transport within the surf zone as measured with a distributed array of upward-looking sonars," J. Acoust. Soc. Am, 109, 133-142, 2001.

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1 Jan 2001

A collaborative, multi-institute experiment called the Scripps Pier Experiment was conducted in the vicinity of the Scripps pier in La Jolla, California, in March 1997 to study the fate of bubbles in the surf zone and the effects of these bubbles on acoustic propagation. This paper discusses data gathered by the Applied Physics Laboratory, University of Washington, using a set of four upward-looking sonars (frequency 240 kHz), which simultaneously measured vertical profiles of acoustic volume scattering from bubbles at four locations. The transport of bubbles via rip currents emerged as an important, though episodic and localized, feature of the acoustic environment in the surf zone. Images of volumetric backscattering strength vs time and depth reveal the episodic events (of increased scattering level) lasting between 5 and 10 min caused by the passage of bubble clouds over the sonar. Time lags for the onset of increased scattering at the four locations are consistent with a seaward velocity of the bubble clouds of order 10 cm/s, and the length scales of these bubble clouds in the seaward direction are inferred to be in the range 50–100 m. The influence of the incoming surface wave field is also discussed.

High-frequency forward scattering from the sea surface: The characteristic scales of time and angle spreading

Dahl, P.H., "High-frequency forward scattering from the sea surface: The characteristic scales of time and angle spreading," IEEE J. Ocean. Eng., 26, 141-151, 2001.

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1 Jan 2001

Forward scattering from the sea surface is discussed in the contest of a forward bounce path, or channel, through which high-frequency sound energy is transmitted. Such a channel might be used in an underwater communication or imaging task. Both time and angle spreading are inherent to the process of forward scattering by a roughened sea surface. Spreading in each domain relates, via Fourier transform, to a conjugate or coherence separation variable, e.g., angle spreading and spatial coherence. The measurement and modeling of time and angle spreading are discussed, with the modeling incorporating the bistatic cross section of the sea surface. A characteristic scale for each spread variable is defined: L for the time spread and αΘh and αΘv for the horizontal and vertical angular spread, respectively. Simplified expressions for these characteristic scales as a function of array acquisition geometry and sea surface conditions are also obtained. Data from two field experiments are discussed, one conducted in shallow waters of 30-m depth, and one conducted in deep, pelagic waters of 4000-m depth. Both experiments utilized frequencies =20 kHz. The role of bubbles in forward scattering is illustrated using measurements from the deep-water experiment. It was demonstrated that bubbles can attenuate the forward-scattered signal, but otherwise have little effect on L and αΘh,v until their concentrations approach those necessary to nearly extinguish the signal scattered from the air/sea interface.

Overview of SAX99: Environmental considerations

Richardson, M.D., et al., including P.H. Dahl, D. Tang, and E.I. Thorsos, "Overview of SAX99: Environmental considerations," IEEE J. Ocean. Eng., 26, 26-53.

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1 Jan 2001

A 1-km2 area located 2 km off the Florida Panhandle (30°22.6'N; 86°38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18–19 m of water, is characterized by 1–2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics.

Monostatic and bistatic scattering by a single bubble near a pressure release interface: Laboratory measurements and modeling

Kapodistrias, G., and P.H. Dahl, "Monostatic and bistatic scattering by a single bubble near a pressure release interface: Laboratory measurements and modeling," J. Acoust. Soc. Am., 108, 2545, 2000.

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1 Nov 2000

Scattering by a single bubble near a flat pressure release interface is investigated theoretically and experimentally. A ray-acoustic interpretation is used to describe the four scattering paths, from source to bubble to receiver. Multiple scattering effects are accounted for using a closed-form solution derived from the multiple scattering series. A bubble with radius a≅425μm is placed on a fine nylon thread, which is practically transparent to sound, at a distance dB from the interface. The primary variable is dB and it ranges from 1a to 100a. Experiments are performed at a frequency of 120 kHz with the transducers arranged in both bistatic and monostatic configuration. Theory and experiment are in excellent agreement, verifying the dominant effect of the four paths in the response of the bubble, with multiple scattering playing a role for kdB<1, where k is the wave number of the medium. In the long-range limit our simulations agree with those of Ye and Feuillade [J. Acoust. Soc. Am. 102, 798-805 (1997)] including the shifting of the bubble's resonant frequency. The dependence of scattering on transducer arrangement, range to bubble, grazing angle, and phase relation among the four paths, vis-à-vis monostatic and bistatic scattering, are discussed.

On bubble-mediated scattering from the sea surface: Modeling, field, and laboratory experiments

Dahl, P.H., "On bubble-mediated scattering from the sea surface: Modeling, field, and laboratory experiments," J. Acoust. Soc. Am., 108, 2492, 2000.

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1 Nov 2000

Bubbles, located just below the air–sea interface, contribute significantly to the observed level of acoustic scattering originating from the sea surface, in contrast, for example, to microwave scattering. Typically, such bubbles are the result of breaking wind-generated waves, and thus their concentration is closely linked to wind speed. In this paper we discuss scattering from bubbles located within the proximity of the sea surface, with emphasis on the effects of the nearby reflecting surface, which gives rise to multiple paths from source to bubble to receiver. The effects of scattering geometry, e.g., as in bistatic forward scattering and monostatic backscattering, are illustrated using field data taken at a frequency of 30 kHz. An interpretive model for the contribution of near-surface bubbles to the apparent scattering cross section per unit area of sea surface is also discussed. This model has both a monostatic and a bistatic form, and, somewhat paradoxically, the bistatic form does not reduce to the monostatic form in the limit of source and receiver co-location. The issue is clarified, however, by examining the constituent pressures associated with the multiple paths, and by well-controlled laboratory measurements that include rough surface effects. These confirm both monostatic and bistatic forms of the model.

The Environmental Acoustics of Two Alaskan Rivers and Its Relation to Salmon Counting Sonars

Dahl, P.H., H.J. Geiger, D. Hart, J.J. Dawson, S.V. Johnston, and D.J. Degan, "The Environmental Acoustics of Two Alaskan Rivers and Its Relation to Salmon Counting Sonars," APL-UW TR 2001, August 2000.

1 Aug 2000

Effects of interaction between two bubble scatterers

Kapodistrias, G., and P.H. Dahl, "Effects of interaction between two bubble scatterers," J. Acoust Soc. Am., 107, 3006-3017, 2000.

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1 Jun 2000

The backscattering of sound from two regularly arranged bubbles is studied theoretically and experimentally. In well-controlled laboratory experiments a bistatic acoustic system is used to interrogate the scatterers, which are placed on a very fine thread at the same distance d from the combined beam axis of the set of transmitting and receiving transducers. The radius of each bubble is 585 μm. The frequency range is 80–140 kHz, and d is varied so that the variable kd spans the range 0.2–21, where k is the acoustic wave number. Scattering calculations are carried out using an exact, closed-form solution derived from the multiple scattering series. Several experiments are performed, and the results are in close agreement with the calculations. It is verified that multiple scattering induces an oscillatory behavior about the exact coherent scattering level, with decreasing amplitude for increasing kd. For interbubble distance 2d ≈ λ/2 the backscattered radiation is maximized, while for 2d<λ/2 the radiation is reduced considerably. These and other effects are discussed.

Inventions

Airborne Acoustic Particle Motion Sound Meter

Record of Invention Number: 48135

David Dall'Osto, Peter Dahl

Disclosure

1 Aug 2017

Pile with Sound Abatement

A noise-attenuating pile comprising a pile driving shoe, an outer tube that engages the pile driving shoe, and an inner member that extends through the outer tube and engages the pile driving shoe, wherein the pile is configured to be installed in sediment or other suitable material by driving the inner member with a pile driver, without directly impacting the outer tube, such that the radial outer tube is substantially insulated from the radial expansion waves generated by the pile driver impacting the inner member. In some piles, one of the inner member and the outer tube are removable after installation. In some piles, a seal is provided in a lower end of the channel defined between the inner member and the outer tube, which may be biodegradable, or may be an inflatable bladder, for example.

Patent Number: 9,617,702

Per Reinhall, Peter Dahl, John Dardis III

Patent

11 Apr 2017

Pile to Minimize Noise Transmission and Method of Pile Driving

Patent Number: 8,622,658

Per G. Reinhall, Peter Dahl

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Patent

7 Jan 2014

A pile and method for driving a pile includes a pile having a structural outer tube, and an inner member disposed generally concentrically with the outer tube. The outer tube and inner member are fixed to a driving shoe. The pile is constructed and driven such that the pile driver impacts only the inner member. The impact loads are transmitted to the driving shoe to drive the pile into the sediment, such that the outer tube is thereby pulled into the sediment. In a particular embodiment the outer tube is formed of steel, and the inner member also comprises a steel tube. In an alternative embodiment one or both of the inner member and the outer tube are formed of an alternative material, for example, concrete. In an embodiment, the outer tube has a recess that captures a flange on the inner member. In an embodiment the outer tube is attached to the inner member with an elastic spring.

More Inventions

Underwater Sound Level Meter

Record of Invention Number: 46351

David Dall'Osto, Per Reinhall, Tim Wen, Peter Dahl

Disclosure

8 Jan 2013

Pile to Minimize Noise Generated During Pile Driving

Record of Invention Number: 8551D

Peter Dahl, Per Reinhall

Disclosure

14 Dec 2009

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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